1. Field of the Invention
The present invention relates to a two-piece steel oil ring for use in internal combustion engines, to a steel wire of a modified cross-section for use as the material of the oil ring, and to a method of producing the steel wire.
2. Description of the Related Art
In general, there are two types of steel oil rings for use in internal combustion engines, namely, a three-piece type steel oil ring composed of a pair of rings having a rectangular cross-section and a spring, and a two-piece type steel oil ring which is rather new and which is composed of a single grooved ring with a modified cross-section and a spring.
Demand for two-piece oil rings in lieu of three-pieces oil rings is increasing in order to reduce the number of parts and, hence, cost. The grooved ring with modified cross-section used as a part of the two pieces oil ring has a cross-section of a complicated shape such as, for example, substantially H-shaped or X-shaped cross-section. In addition, this ring has a comparatively large degree of a modification of cross-section, that is, the ratio of the thickness of the web portion bridging both side portions to the thickness of both side portions is relatively small.
Demand also exists for use of steel alloys having large contents of alloying elements as the ring material, in order to improve resistance to wear, scuffing and sulfuric acid corrosion.
Usually, an internal combustion engine employs a plurality of piston rings including compression rings and oil rings. The compression ring can be produced without substantial difficulty because it has a simple cross-section approximating a rectangle. In fact, compression rings have been shifted to be made of steel alloys having high contents of alloying elements, e.g., 17% Cr type martensitic stainless steel.
Three-piece oil rings usually have a ring member which is generally referred to as "side rail", with the side rail, in most cases, being made of a stainless steel such as SUS 420 J2 (0.36% C-13% Cr type) or a martensitic stainless steel of 0.65% C-13% Cr type. This member also has a simple cross-section approximating a rectangle so that it can be produced by conventional methods such as cold rolling or cold drawing. Examples of the side rail for use in three-piece oil ring are a ring of 0.65% C-13% Cr type stainless steel as disclosed in Japanese Patent Examined Publication No. 61-54862 and 0.55% C-7% Cr type stainless steel as disclosed in Japanese Patent Unexamined Publication No. 61-59066.
Demand also exists for higher degree of alloying of oil ring materials. Resistances to wear, scuffing and corrosion of two-piece oil ring are expected to be appreciably improved by increasing the contents of alloying elements in the ring material. It is, however, extremely difficult to employ, as a material of the ring member of two-piece oil rings with modified cross-section, a steel having higher carbon content than conventional material, e.g., a martensitic stainless steel containing more than 0.8 wt % of C and more than 15 wt % or Cr. More specifically, a thin ring has a complicated cross-sectional shape with large degree of modification from ordinary rectangular shape, so that a heavy plastic working has to be performed when deforming the cross-section. It has been experimentally determined, that cracks tend to be generated in the boundary between the web portion which undergoes strong vertical compression force and flange portions which do not receive comparatively small compression during the plastic working. Furthermore, in order that an oil ring performs expected functions, the ring surface is strictly required to be highly smooth, e.g., 3S or smoother in terms of surface roughness, and to be devoid of defects such as dents or scratches. When conventional cold drawing is employed in the production of an oil ring from an alloy material having high contents of alloying elements, cracks tend to be generated in the boundary between the web and the flanges due to inferior workability of this type of material and due to the large degree of modification of cross-section. In order to prevent such cracking, it is necessary to reduce the amount of the working to be effected per single pass of the cold working and the amount of working per single annealing cycle. Consequently, the number of passes and the number of annealing cycles to be performed before the final product is obtained are increased. The increase in the number of passes and annealing cycles not only increases the risk of damage to the ring material but also raises the production cost. For these reasons, two-piece oil rings could not be commercially produced from a martensitic stainless steel containing more than 0.8 wt % of C and more than 15 wt % or Cr.
The ring member of a two-piece oil ring is produced by modifying the cross-section of a wire into a required shape and then perforating the web portion so as to form a series of plurality of through-holes as oil passage holes. Such perforation can be practically effected only by punching, due to restriction from the production cost and work efficiently. It has proved that such punching is extremely difficult to conduct when the material has high contents of alloying elements.
More specifically, while conventional steel materials having low contents of alloying elements enables punching even when they have been quenched and tempered to the required hardness of HV300 to HV450, punching is materially impossible to conduct for the material having high contents of alloying elements even when the material has been heat-treated to exhibit hardness falling within the above-described range. A method also has been adopted conventionally in which punching is executed subsequently to shaping into the desired cross-section without employing annealing. Such method also cannot be applied to the production from the above-mentioned material having high contents of alloying elements.